This invention relates to color television systems, and is more particularly concerned with signal-processing systems for improving the signal-to-noise ratio of color video signals.
There is a continuing desire and frequent need in the art of generation and transmission of television to improve the signal-to-noise ratio of the video signal. The problem of excessive noise in the video signal is particularly acute in the currently evolving electronic news gathering (ENG) application of television in which a portable camera is taken into the field, often to locations in which the available light is marginal at best, making it necessary to increase the video gain of the camera to obtain an acceptable picture; this increase in video gain results in an attendant increase in the associated noise displayed in the picture. The problem is further compounded in ENG by the fact that the signal from the camera is usually recorded on a small portable recorder for subsequent playback and transmission by a microwave link to a base station. At the base station, prior to actual broadcast of the signal, the video signal may be subjected to two or three levels of editing, all of these steps introducing additional noise of different forms, so that the ultimate signal available for airing is frequently seriously degraded. Even if the signal delivered by the camera were free of noise, the necessary further processing degrades the signal to a degree that the quality of the portrayal on the home receiver is less than would be desired. A general object of the present invention is to provide a system for reducing noise in a color video signal thereby to improve the signal-to-noise ratio and consequently the quality of the displayed television picture.
The present invention takes advantage of the fact that television signals are periodic, whereas noise is aperiodic or random. In the NTSC system of television, the lowest repetition frequency is the frame rate, namely, 30 frames per second, with each frame consisting of 525 lines. For the eye to preserve continuity, any two successive frames must be very much alike, and in fact, if no motion is present in the picture, every frame will be a reproduction of the one that immediately precedes it. If a signal waveform is repetitive, signal-to-noise ratio can often be improved by making use of the redundant information inherent in repetition. Systems employing this technique are generally classified as signal averagers, the principle of which is described in an article by Charles R. Trimble entitled, "What Is Signal Averaging?" appearing in the April 1968 issue of the HEWLETT-PACKARD JOURNAL. Although not concerned with noise reduction in color video signals, the paper describes in a general way the principle of noise reduction by signal averaging.
The principle of signal averaging to achieve noise reduction has been employed in radar applications, two examples of which are described in a paper entitled, "Signal-To-Noise Improvement Through Integration In A Storage Tube", Harrington and Rogers, Proceedings of the IRE, October 1950, and in a paper entitled, "Analysis Of A Comb Filter Using Synchronously Commutated Capacitors", LePage, Cahn and Brown, AIEE Transactions, March 1953. Both papers deal with a class of noise reduction utilizing integration or averaging, but do not address the effect of motion between successive periodic signals on the effectiveness of the noise reduction. Both papers characterize the signal averaging system as a comb filter, many forms of which are now used in television and elsewhere because of their effectiveness in eliminating unwanted noise energy without affecting the wanted periodic signal. The filters described in these two papers are of the recursive type wherein the present signal is added to the sum of a multiplicity of earlier versions of substantially the same signal so as to achieve in effect, an infinite history of the periodic signal. Neither of these papers, however, suggest how the systems might be utilized to reduce noise in a color video signal, and, as has been noted, do not suggest a solution to the problem of motion.
Application of the principle of recursive filtering for noise reduction in television signals is described in a paper by Murray J. Stateman and Murray B. Ritterman entitled, "Theoretical Improvement In Signal To Noise Ratio of Television Signals By Equivalent Comb Filter Technique" published in 1954 in IRE National Convention Record, Volume 2, Part 4. This paper describes how redundancy and knowledge of the past signals can be used to reduce the noise reaching the television screen. On the assumptions that the signals are approximately periodic from frame to frame, and the deviations of the transmitted signal from periodicity are small, and that a serious source of deterioration in picture quality is due to random impulse type noise, by limiting the difference between elemental signals in successive frames to a value consistent with the portrayal of a moderate degree of motion, the noise pulses are attenuated before reaching the television screen. A device suggested by the authors for restraining the incoming television signal includes an amplitude gate in which the present video signal is compared with a signal delayed by one frame period. The amplitude gate passes the present video signal only if it lies within a preset range, that is, if it is within a predetermined range of the previously accepted signal amplitude. When the present video signal lies outside that range, then it is modified in the amplitude gate so that it is no further from the previous signal than the predetermined amount. The modified output signal is then fed to the deflection and video circuits, and also to the one frame delay where it is stored for comparison with the next incoming corresponding signal. The stored signal is not combined with the present incoming signal, but, rather, is compared with the present signal and the present signal modified in some proportion of the difference between the stored and present signals. The amount of stored signal compared with the present signal is always constant, which has the effect of placing a restraint on the integration achieved by the system, which, in turn, has the effect of restraining the motion complexity which may be portrayed. In other words, in the Ritterman and Stateman system a small amount of motion is allowed to take place, the authors recognizing that if more than that amount of motion occurs the portrayal will be severely degraded, as by smearing of the picture. Although the Stateman and Ritterman system will achieve an improvement in signal-to-noise ratio in a black and white television signal, it does not adequately solve the problem of motion and is incapable of reducing noise in a color video signal.
U.S. Pat. No. 3,875,584 discloses a noise reduction system for a color video signal which utilizes filtering of the nonrecursive type in that the present frame of video is summed with one or more preceding frames delayed by one or more frame periods, as by storage on separate channels of a disc recorder. This system deals (somewhat inadequately) with the problem of motion between succeeding frames by simply reducing the number of past frames that are integrated, and does not attempt to reduce noise that may be present in the chrominance component of the video signal. The incoming signal is applied to a comb filter which divides the signal into its luminance and chrominance components, and the luminance component of up to four successive earlier frames are stored in a multichannel disc recorder to enable summing of the luminance component of the present frame with the luminance component of at least one and up to four preceding frames in order to obtain noise reduction in the luminance component. The noise-reduced luminance component is then recombined with the separated and appropriately delayed chrominance component of the present signal to obtain a reconstructed video signal for broadcast or display. While this system has the capability of reducing noise, it by-passes the problem of dealing with noise occurring in the chrominance component, the effect of which is highly visible in the television display and regarded by those versed in the broadcasting art as at least as objectionable as that produced by noise occurring in the luminance component. Thus, the system described in U.S. Pat. No. 3,875,584 is incapable of reducing noise in the chrominance component of the color video signal, it does not satisfactorily solve the motion problem in the respect that its effectiveness in reducing noise when there is significant motion between successive frames, and since it utilizes nonrecursive filtering, the system requires as many storage channels as the manner of signals one desires to sum, an aspect which obviously contributes to the complexity and cost of the system.
Accordingly, it is a primary object of the present invention to provide a system for reducing noise in a color video signal which overcomes the shortcomings of prior systems, and more particularly to provide a system that reduces noise in both the luminance and chrominance components of the signal while more adequately solving the problem of motion than was achievable with prior systems.